The storage system 101 shown in FIG. 1, for example, is known as a storage system that has SAS as its backend (between the controller and the storage device) communication standard. In the explanation that follows, a plurality of elements of the same type may be distinguished by using an element identification number. For example, when distinguishing between a plurality of element As, these elements may be described as “A #x”. “A #x” signifies that the identification number of element A is x. The “x” is an integer of equal to or larger than 0.
The storage system 101 comprises a plurality of storage devices 151, a redundant controller 131 (Controllers #0 and #1), a first communication path 161 to which the controller #0 and the plurality of storage devices 151 are coupled, and a second communication path 163 to which the controller #1 and the plurality of storage devices 151 are coupled.
Each controller 131 is coupled to a host 103 via a communication network 105, and comprises a SAS controller 141 that carries out communications in accordance with the SAS protocol. The controller #0 (#1) receives an I/O request from the host 103. The controller #0 (#1) issues via the SAS controller #0 (#1) an I/O (Input/Output) command targeted at the storage device 151 identified based on this I/O request. Or, the controller #0 (#1) transfers the I/O request from the host 103 to the other controller #1 (#0), and the controller #1 (#0) issues via the SAS controller #1 (#0) an I/O command targeted at the storage device 151 identified based on this I/O request. The second communication path 163 is the redundant route of the first communication path 161. For this reason, each storage device 151 is able to be accessed by way of either the first or second communication path 163.
Each communication path comprises a plurality of SAS expanders (hereinafter, expander) 143 coupled in series. The first initial-stage expander 143 of the plurality of expanders 143 is coupled to the controller 131. Therefore, the controller 131 and the plurality of expanders 143 are in series. The controller 131 and the expander 143, and the expander 143 and the expander 143 are coupled by way of physical links (physical wires).
In a storage system of this type, a discover process is carried out in the backend. In the discover process, a command (a SMP command) that conforms to the SMP (Serial Management Protocol) is issued from the controller 131. The SMP command passes through the same physical link as the physical link that is capable of being used in an I/O (Input/Output) to/from the storage device 151. In accordance with SMP, the physical link through which this command passes is occupied from the time the command is issued from the controller 131 (initiator) to the target device (either the expander 143 or the storage device 151) until a response is returned to the controller 131. Therefore, this physical link is also occupied while the command is being processed. For this reason, the greater the number of SMP commands, the more difficult it is to use the physical link for I/O, and I/O performance is likely to deteriorate as a result.
Patent Literature 1 discloses SSP (Serial SCSI Protocol) as a type of protocol that does not occupy the physical link while the command is being processed. Issuing an SSP command instead of the SMP command in the discover process makes it possible to reduce deterioration in I/O performance.